Pretty sure he's talking about Real Life Trades located in the Donor Forum, as he mentioned more than just bnet credits.
I saw there is a post where people buy and sell battlenet credits, money, gift cards, games? For fg ? Can someone locate me there
You need to go through the process of gaining Donor Forum access if you want to be able to do trades like that. Bnet Credits seems to be available in the bnet section, but for other things such as gift cards, you need the DF. This link will tell you what you need to know if you are interested.